The present invention relates to an apparatus for detecting a focusing state of an optical system and, more particularly, to an apparatus for detecting a focusing state of a convergent lens for converging a laser beam to project the beam to a target surface such as the surface of an optical disk.
Various systems for projecting a laser beam to an optical disk, on which audio, video or another data or information is recorded in pits or another formation, to read out information or data are already known. In such systems, it is required to maintain a convergent lens in a focusing state, i.e., to locate the lens to always be separated by a predetermined distance, i.e., the focal length of the lens from the surface of an optical disk, and to form a beam waist spot on the surface of the optical disk. Therefore, in a system for reading out information or data, there is provided a focusing servo system which has an apparatus for detecting a focusing state and moves a convergent lens along the optical axis of the basis of an output signal from the apparatus to maintain the lens in a focusing state.
An apparatus shown in FIG. 1 is known as an example of an apparatus for detecting a focusing state. In the apparatus for detecting this focusing state, a laser beam reflected from the surface of the optical disk is directed toward a photodetector 2 having first and second photo sensitive regions 2-1 and 2-2 concentrically arranged, and a beam spot is formed on the photo sensitive regions 2-1 and 2-2. When the convergent lens is in a focusing state and a laser beam directed from the convergent lens toward the optical disk forms a beam waist spot on the optical disk, as shown in FIG. 2A, a beam spot 6-1 having a size shown in FIG. 2A is produced on the photo sensitive regions 2-1 and 2-2 of the photodetector 2, the levels of the photo signals respectively generated from the first and second photo sensitive regions 2-1 and 2-2 are equal to one another, and the level of the output signal from a differential amplifier 4 is maintained at substantially zero. When the convergent lens is deviated from the position of the focusing state in a direction away from the optical disk, the laser beam diverged, as compared with the laser beam shown in FIG. 2A, is directed toward the photodetector 2 as shown in FIG. 2B, and a larger beam spot 6-2 is formed on the photo sensitive regions 2-1 and 2-2 as compared with FIG. 2A. Accordingly, the level of the photo signal from the second photo sensitive region 2-2 becomes larger than that of the photo signal from the first photo sensitive region 2-2, and a positive output signal is supplied from the differential amplifier. When the convergent lens is, on the other hand, deviated from the position of the focusing state in a direction for approaching the optical disk, a laser beam converted as shown in FIG. 2C is directed toward the photodetector 2, and a smaller beam spot 6-3, as compared with FIG. 2A, is formed on the photosensitive regions 2-1 and 2-2. Consequently, the level of the photo signal from the second photo sensitive region 2-2 becomes smaller than that from the first photo sensitive region 2-2, and a negative output signal is supplied from the differential amplifier. In a conventional focusing state detecting apparatus, a convergent lens is moved along the optical axis in accordance with an output signal from a differential amplifier 4, and the lens is always maintained in a focusing state.
The conventional apparatus has a following problem. When the distance between a convergent lens and an optical disk is shorter than a predetermined value or when the lens exessively approaches the disk, a laser beam emitted to the photodetector 2, as shown in FIG. 2D, expands and become thicker than the laser beam shown in FIG. 2C. As a result, a relatively larger beam spot 6-4 is formed on the photosensitive regions 2-1, 2-2 as shown in FIG. 2D. Therefore, even if the lens is too close to the disk surface, a differential amplifier 4 generates signals similar to those which may be produced when the lens is at too long a distance from the disk surface. There is the risk that the convergent lens should further approach the disk surface. FIG. 3 shows a relationship between the level of the output signal from the differential amplifier 6 and a distance .DELTA.Z deviated from the position when the convergent lens is in a focusing state. In FIG. 3, a range I shows the level of the output signal from the differential amplifier in the case when the convergent lens is deviated from the position of the focusing state in a direction away from the optical disk. Ranges II and III respectively show the levels of the output signals from the differential amplifier in the case when the convergent lens is deviated in a direction for approaching the optical disk from the position of the focusing state, and partioularly, range III shows the case when the level of the output signal of the differential amplifier becomes positive, irrespective of the case when the convergent lens approaches the optical disk within a predetermined distance from the position of the focusing state. As apparent from FIG. 3, in range III, the convergent lens cannot be located in the focusing state by the output signal from the differential amplifier. Particularly, in a system for optically reading out information, the convergent lens might be located within range III due to a disturbance, such as a vibration, and might collide with the disk surface in the worst case.